Oil well pipes such as tubing and casing used in the drilling of oil or gas wells are typically connected to each other by threaded joints. In the past, the depth of oil wells was typically 2000 to 3000 meters, but more recently, the depth of oil wells may reach 8,000 to 10,000 meters in offshore oil fields and other deep wells.
A threaded joint for oil well pipes is subjected to various forces in its environment of use, including loads such as axial tensile forces caused by the weight of the oil well pipe and the joint member (coupling), pressures due to combined internal and external pressures, and geothermal heat which increases as the depth increases. Therefore, a threaded joint needs to be able to maintain gas tightness without rupture in such environments.
During the process of lowering tubing or casing into a well, a joint which is once fastened for connection often needs to be loosened and then re-fastened. API (American Petroleum Institute) requires that there be no occurrence of galling, which is unrecoverable severe seizing, and that gas-tightness be maintained even if fastening (make-up) and loosening (break-out) are repeated ten times for a tubing joint and three times for a casing joint, which has a larger diameter than a tubing joint.
In order to cope with these requirements, in recent years, a special threaded joint which has a metal-to-metal contact portion without threads (hereunder referred to as an “unthreaded metal contact portion”), in addition to a threaded portion, and which is capable of forming a metal-to-metal seal to improve gas-tightness has widely been used. In this type of threaded joint, typically, a pin is formed on the outer surface of each end of an oil well pipe, and it has an externally threaded portion and an unthreaded metal contact portion. A mating box is formed on the inner surface of a separate, sleeve-shaped coupling member, and it has an internally threaded portion and an unthreaded metal contact portion so as to mate with the corresponding threaded and unthreaded portions of the pin. When the pin is screwed into the box for fastening, the unthreaded metal contact portions of the pin and the box are made to tightly contact each other to form a metal-to-metal seal.
In order to guarantee adequate sealing properties by the metal-to-metal seal of a threaded joint in the environment of an oil well pipe, an extremely high pressure must be applied to the unthreaded metal contact portion during fastening. This high pressure makes it easy for galling to take place. Therefore, prior to fastening, a lubricating grease called a compound grease, which is a viscous fluid at room temperature, is applied to the metal-to-metal contact portion and the threads in order to impart an increased resistance to galling and improved gas tightness.
However, a compound grease contains a large amount of a powder of a heavy metal such as lead, zinc, or copper. When a compound grease applied to a threaded joint is washed off, there is a concern of its causing environmental pollution. In addition, the application of compound grease worsens the work environment and decreases the work efficiency. Thus, there is a need of a threaded joint which can be used without application of a lubricating grease such as a compound grease.
With respect to a threaded joint for which it is not necessary to apply a lubricating grease, JP 08-103724 A1 (1996), JP 08-233163 A1 (1996), and JP 08-233164 A1 (1996) propose a threaded joint in which a solid lubricant-based lubricating coating (hereinafter referred to as “solid lubricating coating”) comprising a solid lubricant such as molybdenum disulfide bonded with a resin such as an epoxy resin is formed on the unthreaded metal contact portion of the pin or box of the threaded joint.
However, it is not easy to form a solid lubricating coating to a uniform thickness along the outline of the threads and the unthreaded metal contact portion of a threaded joint. If the coating has an uneven thickness, an excessively high pressure is required in areas where the lubricating coating is thicker to fasten the threaded joint, thereby increasing the fastening torque, or causing the threads to deform and hence causing galling to occur easily. On the other hand, in areas where the lubricating coating is thinner, poor lubrication or rusting may occur easily.
Even in the case where a solid lubricating coating is formed to a uniform thickness, galling may occur easily under conditions that the joint is fastened and loosened repeatedly or foreign matter is present. The presence of foreign matter takes place, for example, when an oil well pipe is stood upright for connection with a threaded joint, and rust formed on the inner wall of the pipe or blasted grains used for descaling and remaining inside the pipe fall through the inside of the pipe and part thereof adheres to the threads or unthreaded metal contact portion.
A solid lubricating coating has poor ductility and fluidity, and therefore, it tends to easily peel off. Under the above-described conditions, during fastening of a threaded joint, a part of the threads or unthreaded metal contact portions of the joint is subjected to an excessively high pressure, thereby causing plastic deformation to occur locally only in such part of the joint, leading to peeling of the solid lubricating coating to expose the underlying bare metal surface. Then, even if the exposed metal surface is small in area, galling occurs easily.
Another disadvantage of a solid lubricating coating is that its rust-preventing properties are not sufficient to adequately protect the threaded joint from rusting while it is stored prior to use in the field near an oil well. Since rust has a poor lubricity, the formation of rust and the accompanying blistering or peeling of the solid lubricating coating make the torque required for fastening of the joint unstable, thereby often resulting in occurrence of galling or a decrease in the gas tightness achieved by fastening.
On the other hand, in the case of a liquid lubricant such as a lubricant grease or oil which is fluid at room temperature, the lubricant confined in the gaps of the threads or the recesses of the surface roughness of the threaded joint can seep out by the action of pressure applied during fastening. As a result, even if a part of the threaded joint is subjected to an excessively high pressure, the liquid lubricant can extend to that part by seepage, thereby making it possible to prevent galling. This action is called a self-repairing function of a liquid lubricant. In general, the higher the fluidity of a liquid lubricant (the lower the viscosity thereof), the higher is its self-repairing function. Furthermore, a liquid lubricant has good rust-preventing properties.
However, among conventional liquid lubricants, those which have good anti-galling properties and which can be applied to a threaded joint having a metal-to-metal seal portion contain a large amount of heavy metals, as is the case with a compound grease, and hence they produce environmental problems. In addition, to application of a liquid lubricant or lubricating grease makes the surface of the applied threaded joint greasy, and foreign matter such as grit, sand, and dirt may tend to readily adhere to the threads and unthreaded metal contact portions of the joint. Particularly, when an oil well pipe is stood upright for connection, rust and blasted grains fall through the inside of the pipe. At this time, if the surface of the threaded joint is greasy, a considerable part of the rust and blasted grains adhere to the surface. As a result, even with a liquid lubricating grease which is expected to exert a self-repairing function, it is difficult to provide sufficient lubricity, and galling may occur upon repeated fastening and loosening of the joint.